test: run tests even if os.cpus() fails

Currently if the os.cpus() call fails every test will fail. As there is
already a test for os.cpus(), the other tests should run even if the
os.cpus() call fails.

PR-URL: https://github.com/nodejs/node/pull/9616



Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>